Method for manufacturing an object from iron-based alloy by isostatic compression

ABSTRACT

In a procedure in which a body formed of an iron-based alloy is manufactured from powder by isostatic compression, the powder is placed in a flexible container and subjected to isostatic pressure and heat. The isostatic hot pressing is carried out at two temperatures, at one of which the iron-based alloy is at least 25 percent in Alpha -phase and at the other of which the iron-based alloy is in gamma -phase. If the treatment at the temperature for gamma -phase is carried out first, the body is cooled below the temperature level at which it is at least 25 percent in Alpha -phase to prevent retention of the gamma phase and then raised to the temperature at which it is at least 25 percent in Alpha -phase.

United States Patent [191 [11] 3,772,009 Isaksson Nov. 13, 1973 [54] METHOD FOR MANUFACTURING AN 752,687 7/1956 Great Britain 75/221 ()BJECT FROM IRON BASED ALLOY BY 618,293 2/1949 Great Britain.. 75/221 202,202 3/1955 Australia 75/221 ISOSTATIC COMPRESSION Sven-Erik Isaksson, Robertfors, Sweden Allmann Svenska Elektriska Aktiebolaget, Vasteras, Sweden Filed: Oct. 18, 1972 App]. No.: 298,579

Inventor:

Assignee:

Foreign Application Priority Data Oct. 18,1971 Sweden 13138/71 References Cited FOREIGN PATENTS OR APPLICATIONS 2/1959 Canada 75/221 5/1951 Canada ..75/221 Primary Examiner-Carl D. Quarforth Assistant Examiner-B. Hunt Attorney-Jennings Bailey, Jr.

[57] ABSTRACT In a procedure in which a body formed of an ironbased alloy is manufactured from powder by isostatic compression, the powder is placed in a flexible container and subjected to isostatic pressure and heat. The isostatic hot pressing is carried out at two temperatures, at one of which the iron-based alloy is at least 25 percent in a-phase and at the other of which the iron-based alloy is in y-phase. If the treatment at the temperature for y-phase is carried out first, the body is cooled below the temperature level at which it is at least 25 percent in a-phase to prevent retention of the 'y-phase and then raised to the temperature at which it is at least 25 percent in a-phase.

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METHOD FOR MANUFACTURING AN OBJECT FROM TRON-BASED ALLOY BY ISOSTATIC COMPRESSION BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to the manufacture of objects from iron-based alloys using isotatic hot pressing.

2. The Prior Art In the manufacture of objects by using isostatic hot pressing, a powder of the material from which the object is to be manufactured is placed in a pressure-tight flexible container having the same shape as the object. The continaer with its contents is inserted into a pressure chamber where it is subjected to high temperature and high, all-sided pressures. The powder particles will then sinter together with formation of a sintered powder body. As the pressure medium in the chamber an inert gas, generally argon, is used.

The method described can be used when manufacturing objects from different materials, among other things from iron-based alloys such as steel of different kinds. It is known to use the method when manufacturing billets from iron-based alloys for further machining to the desired shape,by means of rolling or forging. In the case mentioned, the isotatic hot pressing is carried out at temperatures when the iron alloy is in y-PhtlSC, generally at a temperature of about l,lC. A frequently used pressure is 1,000 bar. Under these conditions a 100 percent compactness in the hot pressing process is not obtained. Such a compactnessis not obtained until the hot pressed object has been subjected to further machining by means of forging or rolling.

According to the present invention, it has proved possible to obtain a 100 percent compactness in objects of iron-based alloys directly during the isotatic hot pressing process, and therefore the forging or rolling operations can be eliminated as working steps to increase the compactness in the goods. The possibility of avoiding these working steps mentioned makes the present invention particularly suitable for forming objects with complicated shape, for example gear cutters, cylinders, dies, and for forming objects having such a shape that machining by means of forging or rolling is impractical, for example tubular parts.

SUMMARY OF THE INVENTION The present invention relates to a method for manufacturing objects from iron-based alloys, a powder of the iron-based alloy being placed in a flexible container and subjected to isostatic hot pressing with formation of the object in the form of a sintered powder body, characterised in that the isostatic hot pressing is carried out at two temperature levels with the iron-based alloy at least up to 25 percent in a-phase at the one temperature level and in 'y-phase at the other temperature level.

The hot pressing at the temperature level with the iron-based alloy at least up to 25 percent in az-phase is preferably carried out before the hot pressing at the temperature level with the iron-based alloy in 'y-phase, but can also be carried out after the last-mentioned hot pressing.

By iron-based alloy is meant an alloy which contains at least 50 percent by weight of iron. Examples of such iron-based alloys are ordinary steel, high-speed steel, tool steel, toughened steel and stainless steel. The temperature level at which at least 25 percent of the ironbased alloy is in a-phase and the temperature level when the iron-based alloy is in -y-phase varies with the composition of the iron-based alloy. 1n the following, examples are given of iron-based alloys with different compositions in percentages by weight (the names within brackets are Swedish designations):

Ordinary steel (SIS 1240) 0.1 C, max. 0.3 Si, 0.4

% Mn, rest Fe.

High-speed steel (SIS 2722) 0.85 C, 0.2 Si, 0.3 Mn, 4.2 Cr, 5.0 M0, 6.4 W, 1.9 V, rest Fe.

High-speed steel (S 30 ISTORAI) 1.2 C, 0.2 Si, 0.3 Mn, 4.2 Cr, 5.0 M0, 6.4 W, 10.3 Co, 3.4 V, rest Fe.

Tool steel (SIS 2310) 1.55 C, 0.3 Si, 0.5 Mn,

12.0 Cr, 0.8 M0, 0.9 V, rest Fe.

Toughened steel (SIS 2541) 0.35 C, 0.25 Si, 0.65 Mn, 1.4 Cr, 1.4 Ni, 0.20 Mo, rest Fe.

Stainless steel (SIS 2338) 0.08 C, 0.5 Si, 1.2 Mn, 17.5 Cr, 10.0 Ni, 10 x C-content Nb, rest Fe.

For all these alloys the temperature at which at least 25 percent of the alloy is in oz-phase lies at 700 950C, and the temperature at which the alloy is completely in y-phase lies at 950C -l,225C.

The lower temperature limit during the hot pressing, when at least part of the iron-based alloy is in a-phase is kept, according to the invention, preferably at 700 C also for other alloys than those noted above. The upper temperature limit during the hot pressing with the ironbased alloy in y-phase is correspondingly kept preferably at 1,225C for other alloys also. The time for the hot pressing with a-phase present is at least 1 hour, preferably 1-10 hours, and the time for the hot pressing in 'y-phase at least one-half hour, preferably r'z--l0 hours.

A possible explanation of the favourable result obtained according to the present invention is that the diffusion of the different substances in the powder at the grain boundaries will be considerably greater if part of the sintering is carried out in a-phase or with a-phase present than if it is carried out completely in 'y-phase. This results in the product aquiring a more homogeneous structure. Because of the higher temperature level, the sintering in 'y phase results in an efficient plastic compression of the powder so that a complete compactness in the product is obtained. The isostatic hot pressing in a-phase or with a-phase present and the isostatic hot pressing in 'y-phase are preferably carried out in immediate connection with each other. If the hot pressing in oz-phase is carried out first, the temperature is increased after the treatment in a-phase is finished directly to the temperature required for -y-phase. If the hot pressing in y-phase is carried out first, the temperature is first lowered to a value below that at which the treatment with a-phase present is carried out, in order to prevent retention of -y-phase, before it is increased to the temperature required for the treatment with a-phase present.

The pressure during the isostatic hot pressing according to the present invention reaches preferably 7003,000 bar. I

The particle size of the powder is below 1,000 microns, preferably between 50 microns and 800 microns. The oxygen content of the powder should be low, 200 ppm at most.

It is advantageous to subject the powder to an isostatic cold pressing for formation of a coherent powder body before it is subjected to the isostatic hot pressing described. The powder is then enclosed in a flexible container which is preferably the same as is used later during the isostatic hot pressing and which has the same shape as the object to be manufactured. After evacuation and sealing the container is inserted for the isostatic cold pressing into a pressure chamber where it is subjected to a high, all-sided pressure, preferably at least 1000 bar. The powder is then compresseduniformly in all directions so that the powder body formed becomes a somewhat smaller copy of the container.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further explained by describing an example or embodiment with reference to the accompanying drawing in which FIG. 1 shows schematically an for manufacturing objects by means of the method according to the present invention,

FIG. 2 the upper part of a powder-filled flexible container,

FIGS. 3 and 4 a press for isostatic cold pressing of powder to a coherent powder body,

FIG. 5 a furnace for isostatic hot pressing of the powder body mentioned to a finished object,

FIGS. 6 and 7 curves showing the temperatures as a function of the time during the isostatic hot pressing according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the figures, l designates a storage container for powder of a high-speed steel consisting of S 30 (STORA), the composition of which has been indicated above, and 2 designates a rotatable table which can be turned stepwise. Close to the table is a store of flexible containers 3, for example for ordinary steel consisting of SIS 1240 (composition given above). On the table an empty container 3a has been placed which by turning the table 2 in steps is moved between a number of different stations. Opposite the storage container 1 is a container 3b in a filling station where it is filled with powder from the storage container 1 through a hose or pipe 64. At the next station is a store of lids 4 with a pipe connection 5 and welding equipment 6. Here a container 3c filled with powder 9 if provided with a lid 4 which is welded to the wall 7 of the container 3 by means of a welding seam 8, as shown in FIG. 2. The container 3d is shown at a transfer station from which it is transferred to a testing station where the tightness of the container is checked. There are three containers 3e at this station. The checking equipment is designated 10.

In the pressure equipment 11 the filled container is isostatically compressed by being inserted in a pressure chamber and subjected to high isostatic pressure. The equipment 1 1, which is further described in connection with FIGS. 3 and 4, consists of a high pressure cylinder 12 supported by a stand 13 and a movable press stand 14 which runs on rails 15. The press stand 14 is of the type having two yokes and two spacers, held together by a pre-stressed strip sheath. It is carried by two pairs of brackets 16 in which the shafts 18 of the transport wheels 17 are journalled. An electric motor 19 drives the shaft 18 of one of the wheel pairs by way of a a gear drive in one of the brackets 16. The transport wheels run on the rails 15. The cylinder 12 is provided with two end closures 20 and 21 projecting into the cylinder, the lower one being suspended and vertically movable to a limited extent in the cylinder 12, whereas the upper one can easily be lifted for charging and emptying the container. The container 12 is provided at its ends with flanges 22 and the stand 13 with brackets 23 having holes through which a rod 24 passes. During charging and emptying the stand is at a little distance from the press cylinder, as shown in FIGS. 1 and 3. After charging and positioning the upper end closure 21, the stand is moved with the help of the motor 19 above the cylinder 12 so that their centre lines coincide, after which pressure medium in the form of oil is introduced into the pressure chamber. The axial pressure operating on the end closure is taken up by the press stand. After the compression process, the end closures are again moved to their inner positions and the stand is moved away from the cylinder 12 so that the high pressure chamber can once again be opened, emptied and charged with a new filled container to be pressed. Beside the pressure equipment 11 there are shown two finished powder bodies 3f enclosed in their containers.

After compression the powder bodies are heated at atmospheric pressure while being degassed simultaneously by means of vacuum suction. Usually the heating is carried out in two stages with a pre-heating stage to a temperature of 550C during which the degassing is carried out and then further heating with a completely sealed container to the temperature required for the isostatic hot pressing. FIG. 1 shows a group of pre-heating furnaces 25. The powder bodies in the furnaces are connected to a vacuum pump 27 by way of pipe connections 5 and a conduit 26 which passes through the lids or roofs of the furnaces 25. After the pre-heating, the opening in pipe connection 5 in the lid 4 of the container is closed and the powder body is transferred to a second pre-heating furnace. There is therefore a group of pre-heating furnaces 30 in the equipment, where the temperature can be raised to the level suitable for the isostatic hot pressing. The furnaces may be of conventional type, for example electric resistance furnaces. The auxiliary equipment of the furnaces is designated 31. After heating the product is transferred to a furnace 32 for isostatic hot pressing. A group of two furnaces 32a and 32b is shown in FIG. 1. These furnaces are further described with reference to FIG. 5. The furnaces are of the type shown in Swedish Pat. No. 315,085 and are thus charged from below. The furnace comprises a furnace chamber which is enclosed in a pressure chamber. This pressure chamber consists of a high pressure cylinder 33 of the type constructed of a tube 34 and a surrounding pre-stressed strip sheath 35, an upper end closure 36 and a lower end closure 37. The cylinder is suspended in a stand 38. The upper end closure 36 is permanently held in the cylinder and is provided with a channel 39 for the supply of pressure medium in the form of argon, helium or nitrogen and a channel for an electric cable 40 to feed electric heating elements 41 and to obtain measuring values from the thermoelement. Above the end closure 36 is a plate 42 with an output for the cable 40. In the upper end closure 36 are an insulating sheath 43 and an insulating lid 44 which divide the actual furnace chamber 45 from the inner wall of the tube 34 and the lower surface of the end closure 36. Furthermore, the heating elements 41 are suspended in the upper end closure 36. In the lower part of the tube 34 is a ring 46 projecting permanently into the tube. The lower end closure 37 is provided with a bracket 47 and a guide 48 and is arranged vertically slidable and turnable on a guide 49. Lowering and raising is done with the help of an operating cylinder 50 attached on the stand, the operating rod 51 of which is connected to the guide. The furnaces unit also includes a movable press stand to take up the forces operating on the end closures. This press stand is also of the type having two yokes 53 and 54, two spacers 55 and a strip sheath 56 holding them together. The stand is provided with brackets 57 to journal wheels 58 running on rails 59. On the lower end closure is a cylinder 60 of insulating material. On this stands a billet 61. During the isostatic hot pressing the stand 52 is moved in over the high pressure chamber, during emptying and charging the stand is a little distance away from the high pressure chamber so that the lower end closure can be lowered and turned, as can be seen in FIGS. 1 and 5.

1n the furnace 32 the powder body with the enclosing container is first subjected to a temperature at which the high-speed steel to at least 25 percent is in a-phase and then to a temperature at which the highspeed steel is in 'y-phase, as illustrated in FIG. 6, or first to a temperature at which the high-speed steel is in -yphase and then to a temperature at which the high-speed steel to at least 25 percent is in a-phase, as illustrated in FIG. 7.

According to FIG. 6, the powder body is pre-heated during the period t in the furnaces 25 and 30 to the temperature T n1 at atmospheric pressure. T a is a temperature at which at least 25 per cent of the iron alloy is in a-phase. After this the isostatic hot pressing is carried out in the furnace 32 at the temperature T 24 and a pressure of 700-3,000 bar during the period t a The temperature is then raised during the period t t to the temperature T a which is the temperature at which the iron alloy is in 'y-phase, and the isostatic hot pressing is carried on at the temperature T during the period t a After this the sintering is completed and the hot compressed product finished. During the periods t u and t a a pressure of 700-3,000 bar is maintained. For the high-speed steel mentioned in the example, T a may be 750 950C, T a 1,000

1,200C, t a 1 5 hours, t a l 5 hours, t 3 hours and t1, 1 hour. A suitable pressure during the isostatic hot pressing is 1,000 bar.

According to FIG. 7, the powder body is pre-heated during the period t to the temperature Ta at atmospheric pressure. After this the isostatic hot pressing is carried out at the temperature T a and a pressure of 700-3,000 bar during the period t a. The temperature is first lowered to the temperature T ,l in order to prevent retention of the y-phase during the continuing isostatic hot pressing at the temperature T a which takes place during srqtips (a te smwsrbor rare this has been heated from the temperature T t to T a The time for lowering the temperature from T 24 to T H- and for raising the temperature from T M- to T a is designated t,,,. During the periods t,,, and t 24 a pressure of 700 3,000 bar is maintained. After the period T a the sintering is completed and the hot compressed Product n hs Fq t shis -spesd ste mentioned in the example, T may be 750-950C, T 4 1,000-1,200C, 2,, 1- 5 hours, t a l- 5 hours, T 650C, t, 3 hours and r 2 hours.

I claim:

1. Method for manufacturing objects from iron-based alloys, in which a powder of the iron-based alloy is placed in a flexible container and subjected to isostatic hot pressing under formation of the object in the form of a sintered powder body, which comprises carrying out the isostatic hot pressing at two temperature levels, one level in which the iron-based alloy is at least up to 25 percent in a-phase, and the other level at which the iron-based alloy is in 'y-phase.

2. Method according to claim; 1, which comprises carrying out the isostatic hot pressing first at the temperature level in which the iron-based alloy is at least up to 25 percent in a-phase and then at the temperature level in which the iron-based alloy is in y-phase.

3. Method according to claim 1, which comprises carrying out the isostatic hot pressing first at the temperature level in which the iron-based alloy is in 'y-phase and then at the temperature level in which the iron-based alloy is at least up to 25 percent in a-phase, and which includes lowering the temperature after the treatment at the first-mentioned temperature level to a value below the last-mentioned temperature level and thereafter raising it to the last-mentioned temperature level. 

2. Method according to claim 1, which comprises carrying out the isostatic hot pressing first at the temperature level in which the iron-based alloy is at least up to 25 PERCENT in Alpha -phase and then at the temperature level in which the iron-based alloy is in gamma -phase.
 3. Method according to claim 1, which comprises carrying out the isostatic hot pressing first at the temperature level in which the iron-based alloy is in gamma -phase and then at the temperature level in which the iron-based alloy is at least up to 25 percent in Alpha -phase, and which includes lowering the temperature after the treatment at the first-mentioned temperature level to a value below the last-mentioned temperature level and thereafter raising it to the last-mentioned temperature level. 